In an optical transmission technology, it is known that a technology for measuring (may be also referred to as “monitoring”) an optical signal to noise ratio (OSNR) indicative of the ratio of a noise component to signal light. For example, documents D1 and D2 listed below disclose methods of calculating a noise component.    D1: U.S. Pat. No. 8,285,148    D2: Markus Mayrock and Herbert Haunstein, “Monitoring of Linear and Nonlinear Signal Distortion in Coherent Optical OFDM Transmission”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 16, Aug. 15, 2009, p. 3560-3566.
In the calculation methods disclosed in D1 and D2, an inverse SNR of a measurement channel is measured by using a coherent optical receiver. In addition, the transmission optical power of the measurement channel is increased by k times on an optical transmission side, and an inverse SNR of the measurement channel is measured. Further, an inverse SNR of another channel different from the measurement channel is measured. Then, based on the results of the measurements, noise components are calculated in order of a cross-phase modulation (XPM) component, a self-phase modulation (SPM) component, and an amplified spontaneous emission light (ASE noise) component. Here, the XPM component and the SPM component are examples of a non-linear noise component.
In the calculation methods described above, the coherent optical receiver of the optical reception side needs to obtain information of “k” corresponding to a power level ratio at the time when the transmission optical power (may be referred to as the “optical fiber launched power”) is increased by k times on the optical transmission side. Accordingly, a configuration to obtain or calculate the non-linear noise component and the ASE noise component with respect to the signal light may be complicated. Thus, the cost of the configuration may be increased.